Sintered photoconducting photocells and methods of making them



Oct. 13, 1959 G. s. BRlGGs 2,908,594

SINTERED PHOTOCONDUCTING PHOTOCELLS AND METHODS OF MAKING THEM Filed March 19, 1957 ZEW IN VEN TOR.

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United States Patent O SINTERED PHOTOCONDUCTING PHOTOCELLS AND lVIETHODS F MAKING THEM George S. Briggs, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application March 19, 1957, Serial No. 647,153 8 Claims. (Cl. 117-201) This invention relates to sintered photoconducting photocells and particularly, but not necessarily exclusively, to improved photocells which are particularly useful in headlight dimming systems for automobiles, and to improved methods of making them.

Sintered photoconducting layers are described by S. M. Thomsen in U.S. Patent No. 2,765,385, issued October 2, 1956, and S. M. Thomsen and R. H. Bube in The Review of Scientific Instruments, High Sensitivity Photoconductor Layers, vol. 26, No. 7 (July 1955), pages 664-665. These layers comprise generally a substantially continuous polycrystalline layer of interlocked photoconducting crystals selected from the group consisting of selenides, suldes, and sulfo-selenides of cadmium having incorporated therein activator proportions of halide and silver or copper. These sintered layers are prepared by coating a desired mixture of raw materials on a substrate and then sintering the coating to recrystallize the major ingredientin a molten solvent, to incorporate the desired impurities therein, and to evaporate the solvent. Photocells, which are cheaply and easily prepared by applying electrodes to the sintered layer, may possess photosensitivities approaching that of single crystals of corresponding compositions.

Photocells of the sintered type, which are desired for uses such as in automobile headlight dimming systems must have the following characteristics. First, they must operate as a switch at battery voltage (l2 volts or less). Second, they must pass sucient photocurrent for switching in response to (a) the headlights of approaching automobiles at about 1000 feet or (b) the taillights of a preceding automobile at about 150 feet. Third, they must be capable of faithfully repeating the switching operation rapidly. This means than-upon return to darkness after excitation illumination, the photocells must recover to at least 50% of their photocurrent within 90 seconds upon a succeeding illumination. Fourth, their electrical properties must be stable over long periods of time. Fifth, their electrical properties must be stable over a wide range of operating temperatures.

An object of this invention is to provide improved sintered photoconducting layers and improved photocells of the sintered type.

A further object is to provide improved processes for preparing sintered photoconducting layers.

Another object is to provide improved photocells of the sintered type which are particularly useful? for example, in automobile headlight dimming systems.

The invention herein is based at least in part on the discovery of the following controlled steps, each of which imparts to previous sintered photoc'ells improved properties; andin combination, imparts the preferred combination of characteristics required for use in automobileV headlight dimming systems.

l) The sintered layer must be prepared from cadmium selenide, which has a preferred spectral sensitivity and an unusual speed of response with respect to similar sulfidecontaining sintered/layers. A

(2) The raw batch for preparing the sintered Y'layer contains between 3 to 7 parts by weight of recrystallizing solvent, preferably cadmium chloride, per 100 parts of cadmium selenide.

(3) The raw batch for preparing the sintered layer contains 0.0001 to 0.0020 part by weight copper impurity per 100 parts cadmium selenide. Increased copper content reduces the overall sensitivity of the cell.

` perature in about 4 to 8 minutes.

(4) The raw batch is coated on a substrate which is subject to a minimum of chemical attack during tiring. The substrate is preferably a dense, high-fired, highalumina ceramic.

(5) The coated substrate is heated to its firing tem- Longer periods result in excessive loss of recrystallizing solvent and shorter periods result in an unstable process.

(6) The coated substrate is red at 5301-20o C. for about 10 to 14 minutes in a restricted volume of stagnant air. These critical processing parameters impart higher sensitivity and faster recovery to the cell.

(7) The tired coating is maintained free of moisture throughout its life. Exposure to moisture, even normal atmospheric humidity for short periods of time, deteriorates the photoconducting properties of the cell.

According to the invention:

(a) The preferred sintered photcconducting layers are prepared by coating a dense, high-tired, high-alumina substrate with the following mixture of raw `materials in parts by weight:

parts cadmium selenide 3 to 7 parts cadmium chloride 0.0001 to 0.002 part copper, as a salt thereof.

The coated substrate is heated to 510 to 550 C. in about 4 to 8 minutes in a restricted volume of stagnant air. Upon cooling, tin or gold electrodes are applied to the sintered photoconducting layer, preferably by vacuum evaporation. The electroded layer is then mounted and sealed in a transparent, moisture-proof envelope, preferably a glass bulb, to produce the improved photocells herein.

The invention is more fully described in the following detailed description when read in conjunction with the drawing in which:

Figure 1 is a sectional, plan view of a preferred photocell of the invention viewed along line 2-2 of Figure 2 illustrating the interdigital electrodes of the preferred photocells,

Figure 2 is a sectional, elevational view of the preferred photocell of the invention viewed along line 1 1 of Figure 1,

Figure 3 is a curve illustrating the relative spectral sensitivity of the photocell of Figure 1 for equal values of radiant flux at wavelengths between 3500 and 10,000 A.,

Figure 4 is a family of curves illustrating the currentvoltage characteristics at various light levels of the photocell of Figure 1 at 25 C.,

Figure 5 is a family of curves illustrating the rise characteristic at various light levels of the photocell of Figure 1, and

Figure 6 is a family of curves illustrating the decay characteristic at various light levels of the photocell of Figure l.

Similar reference characters are applied to similar elements throughout the drawing.

EXAMPLE 1 Referring to Figures 1 and 2, a preferred photocell of the invention comprises a substantially continuous polycrystalline layer 23 of interlocked photoconducting crystals of cadmium selenide containing activator proportions of chloride and between 0.0001 to 0.002 part copper per 100 parts cadmium selenide, on a dense, high-fired, highalumina substrate 21. Connector wires 29 are wedged in holes in the substrate 2d and a pair of tin electrodes 25, in interdigital configuration, are evaporated on the connector wires 29 and the sintered layer 23 to form a substantially uniform gap therebetween of relatively long length. The assembly is supported on lead wires 33 and sealed in a yglass envelope 35 having an atmosphere of dry air at normal atmospheric pressure.

The sintered photoconducting layer 23 is prepared by the following procedure. A substrate 21 comprises a circular disc about 0.610 inch in diameter and having two holes therein about 0.032 inch in diameter about 0.515 inch apart along a principal diameter of the disc.` The substrate 21 is a dense, high-fired, high-alumina ceramic, such as marketed by the American Lava Company, Inc., Chattanooga, Tennessee, under the tradename Alsimag 576. The substrate 2l is cleaned with dilute nitric acid and rinsed 'with demineralized water, dried, and then fired at 800 C. for about 2 hours in air. The tired substrate is cooled and stored in a container until ready for use.

A raw batch is prepared comprising: Y

Cadmium selenide 100 grams Cadmium chloride 5 grams Copper as cupric chloride 0.001,6 gram Demineralized water about 200 ml.

The foregoing mixture is placed in an agitator, preferably a Waring blendor, and mixed for about 5 minutes until a uniform consistency is obtained. rfhe mixture is then sprayed, on one side of the substrate 21, with an air pressure of about 20 lbs. per square inch to a weight of about 4.5 milligrams per square centimeter. This is approximately 9 milligrams per substrate. Several substrates may be sprayed at one time as a matter of convenience. The sprayed coatingT is then dried in air.

Upon drying, three coated substrates 2l are placed on a borcsilicate glass plate and covered with a standard size E chemical porcelain crucible cover. The covered group of coated substrates is then heated up `to about 530 C. in about 6 minutes, fired at 530 C. for about l2 minutes and then cooled to 100 C. in 30 minutes. The Crucible cover is left in place throughout the process. Upon cooling the substrates are removed from the furnace and protected from moisture including the humidity of normal air.

The sintered photoconducting layer 23 produced in Example 1 may be incorporated into a photocell by the following procedure. A pair of nickel connector wires 29, approximately 0.035 inch in diameter, are etched in acid to provide a taper thereto. The tapered connector wires are washed in water, rinsed in acetone and then dried. The connector wires 29 are inserted into the holes in the substrate 21 from the lback of the substrate and pulled through until the wire breaks. The connector wires 29 are then cut off 'as close to the sintered surface as possible and the portion extending from the back of the substrate is trimmed to 57s of an inch. AV clean flat mask (not shown) is placed over the sintered layer 23. The mask has openings which will allow the electrode metal 25 to deposit in a predetermined pattern on the sintered layer 23. Tin metal is then evaporated from an aluminum oxide coated electric heater cone until an opaque coating has been deposited. The evaporation is carried out in a vacuum of about -5 millimicrons at room temperature in about 4 minutes. The sintered layer 23 is about 10 inches from the heater cone. After evaporating the tin, the substrate is cooled for about two minutes and the vacuum is broken. The substrate is removed from the evaporation apparatus and a droplet of silver paste 27 is placed over the electrodes 25 Where the connector wires 29 come through the sintered surface 23. The droplet improves the mechanical strength of the structure.

As shown in Figure 1, the electrodes 25 comprise an interdigitated structure, wherein the gap is substantially uniform and preferably of about 0.030 inch and of relatively long length.

The connector wires 29 are bent parallel to the surface of the back of the substrate 21. A stem assembly is provided comprising flange tubing 41, exhaust tubing 43 and stem leads 33, which extend through the flange tubing 41 parallel to one another and about 3 millimeters beyond the flange tubing 41. he stern leads 33 and connector wires 29 are spot welded at about right angles to one another.

Following welding, a glass cover 35 is joined to the stem tubing 41 along bead 37. The assembly is lled with dry air and then sealed at the exhaust tubing 43. lf desired, the completed cell may be mounted on a conventional base or a special base made therefor. The finished tube is about a half inch in diameter and stands about 1 inch high.

The cell of Example l has the electrical characteristics illustrated in Figures 3 to 6. As shown in Figure 3, the spectral sensitivity of the cell substantially matches the spectral range of tungsten lamp of automobile headlights, and also has a substantial red sensitivity for automobile taillights. As shown in Figure 4, the cell has a substantial operating range below l2 volts in which a substantial photocurrent is passed at relatively low light levels. As shown in Figures 5 and 6, the photocurrent of the cell rises to its full value after excitation in less than a second over a wide range of light levels, and then decays to its stable dark condition after removal of excitation in less than a second over a wide range of light levels. When incorporated into an automobile headlight dimming system, the cell will operate a switch to dim headlights with about 0.01 foot candle of excitation, which is the brightness of ordinary headlights at about 1000 feet or taillights at about 150 feet. The cell will operate the switch back to the bright position when the excitation falls below about 0.001 foot candle.

The substrate-The substrate 21 should be substantially inert to the mixture of raw materials throughout the entire processing. Dense, high-red ceramic materials preferably having an alumina content greater than by weight are most desirable. Examples of desirable substrate, dense, high-ired ceramic materials are:

(l) Alsimag 614, American Lava Co., Chattanooga, Tenn., containing about A1203, and 5% flux.

(2) Alsimag 576, American Lava Co., Chattanooga, Tenn., containing about 85% A1203.

(3) Alsimag 475, American Lava Co., containing chiefly zirconium silicate crystals.

(4) Alsimag 35, American Lava Co., containing chiefly clinoenstatite crystals.

The substrate may be of any desired shape and thickness. Flat discs are convenient for processing and mounting. Prior to processing, the substrate 21 is heated in air at an elevated temperature for an extended period to relieve the substrate of adsorbed moisture and gases. Temperatures between 600 and 900 C. for an hour are convenient.

The mixture of raw materials.-Cadmium selenide is a necessary ingredient of the sintered photoconducting layers herein. The cadmium selenide preferably has a high degree of purity; preferably spectroscopically pure. The cadmium selenide preferably is fine grained with an average particle size of about one micron. Cadmium selenide imparts to the cells herein the desired spectral sensitivity and an unusually fast speed of response compared with sulfide-containing cells.

Cadmium chloride is introduced into the mixture to act as a molten solvent and recrystallizing medium for the cadmium selenide during firing. Other materials which may be used are cadmium bromide and cadmium iodide. At the conclusion of the firing step, substantially all of the molten solvent evaporates leaving behind the intera,

locked, recrystallized cadmium selenide with the desired impurities incorporated therein. A very small amount of chloride ions is believed to be incorporated in the r'ecrystallized cadmium selenide and to occupy selenide lattice sites as a substitutional impurity. Previous publications indicated use of about l parts by weight of cadmium chloride per 100 parts of cadmium selenide and placed no criticality on the proportion of cadmium chloride. The sintered layers herein are prepared with a lower proportion of cadmium chloride in the critical range between 3 and 7 parts cadmium chloride per 100 parts cadmium selenide. This reduction in proportion of cadmium chloride increases the speed at which the photocell recovers to equilibrium in darkness following removal of excitation illumination.

An activator proportion of copper, as a salt thereof, is included in the mixture of raW materials. The copper salt may be copper chloride, copper brom-ide, copper acetate, copper nitrate or copper formate. The copper salt is conveniently added as an aqueous solution thereof to the mixture of raw materials. Copper ionsV are believed to enter the cadmium selenide crystals in the monovalent form and occupy cadmium lattice sites as a substitutional impurity. Copper reduces the light and dark conductivity, which is the reverse of the elect of the incorporated chloride impurities.

Previous publications have stated that copper and other metals may be used as an impurity in proportions over a very wide range. The photocells herein are prepared only with copper, which has been found to be critical, and in lower proportions in the critical range between 0.0001 to 0.0020 part by weight of copper per 100 parts cadmium selenide. As a result of this reduction in proportion of copper to the narrow range herein, the cells herein have increased photosensitivity over previous cells.

The mixture of raw materials is coated on the substrate by any convenient method for accomplishing this. The coating 'should be as dense as possible. The coating should weigh between 4 to 5, preferably 4.5, milligrams per square centimeter. The coating is dried in air and is then ready for tiring.

Firing procedure-The firing procedure aifects the electrical characteristics of the sintered Vlayers herein. 'I'he sintered layers herein are prepared by heating up and iring at a. lower controlled temperature and at a slower controlled rate than previously. Specifically, the coated substrate is heated to a temperature of about 510 to 550 C. in about 4 to 8 minutes. The higher temperatures are preferred for the shorter times. The coated substrate is then fired at a temperature of 510 to 550" C. for about to 14 minutes. The higher temperatures are preferred with the shorter times. These times and ternperatures are critical, imparting faster recovery and higher photocurrent to the sintered layers herein.

The firing atmosphere is a restricted volume of stagnant air. This may be obtained by any convenient means of confining the air circulation around and above the coating during firing. Unlike previous processes, oxygen is necessary in the tiring atmosphere of the processes herein to impart faster recovery and Ihigher photocurrent to the sintered layers herein.

The cooling rate also affects the recovery of the sintered layers described herein, although not so critical as some of the other parameters. It has been found that cooling the sintered layer immediately after ring to about 100 C. in 30 minutes results in faster recovery than similar layers cooled to 100 C. in 12 hours.

Table 1 shows the effect of varying the rise time (time for heating to firing temperature) on the electrical properties of the photocells herein. The layers were cooled at 1 C. per minute. The cells were tested at 9 volts. Table 2 shows the effect of Varying the cooling time on the electrical properties of the photocells herein. I'he temperature rise time was 5 to 8 minutes. The cells were tested at 9 volts.

Table 1.-Eect of temperature rise time on electrical properties of sintered surface Temperature Rise Time (room Temperature Rise Time (room to furnace temperature) 1l to to furnace temperature) 5 to 14 minutes 8 minutes.

Current Current Current Current in ya. at in na. at Percent in ya. at in ua. at Percent 0.0 ft. 0. 01 ft. recovery 0.0 ft. 0.01 it. recovery candles candles candles candles Table 2.-E)fecl of rate of cooling cycle on electrical properties of sintered surface The plz'0t0cell.-The electrical characteristics of the photocells herein deteriorate in the presence of moisture. Accordingly, following firing, tected from moisture and humidity until sealed in a moisture resistant container.

Electrodes are applied to the sintered layers herein by any of the well-known methods of electroding. The preferred method is by evaporation of the preferred metals in a vacuum. The preferred metals are tin, indium and gold. The electrodes may be in any configuration but the interdigitated structure is preferred since it provides a maximum gap length, and therefore a maximum photocurrent, for a given photocell area.

The electrodes are connected to connector wires 29. One method, which provides a mechanically strong Stnucture, is to taper the connector wire as by etching the wire. The wire is drawn through a hole in the substrate until it wedges in the hole. The wire may be drawn through the front or the back of the substrate. The excess lead wire is then trimmed.

The mounted sintered layer is sealed into a moisture tight container. Glass, or metal with a glass window, :may be used. The sealed photocell is maintained at about normal atmospheric pressure. A dry atmosphere of air, nitrogen, oxygen and argon are satisfactory. Evacuation of the container adversely alfects the electrical characterstics of the sintered layer whereby the photocells become less stable with respect to both temperature and time.

EXAMPLE 2 Follow the procedure of Example 1 except substitute gold for tin in the electrodes.

EXAMPLE 3 Follow the procedure of Example 1 except use the following mixture of raw materials:

the layers should be pro.

EXAMPLE 4 Follow the procedure of Example 1 except use the ollowing mixture of raw materials:

Cadmium selenide grams 100 Cadmium bromide do 6 Copper, 4as copper chloride do 2 07.0015

Water ml 200 EXAMPLE 5 Follow the procedure of Example 1 except use the following mixture of raw materials:

Cadmium selenide grams-- 100 Cadmium chloride do 5 Copper, as copper acetate do 0.0019

Water ml 200 EXAMPLE y6 Follow the procedure of Example 1 except use a substrate of a dense red ceramic of Alsirnag 614.

What is claimed is:

l. A process for producing a photoconducting layer comprising coating a substrate with an intimate mixture in parts lby weight of:

100 parts cadmium selenide 3 to 7 parts cadmium chloride 0.0001 to 0.002 part copper as a copper salt,

and tiring said coating a restricted volume of stagnant air until `said material is sintered.

2. A process :for producing a photoconducting layer comprising coating a substrate with an intimate mixture in parts by weight of:

100 parts cadmium selenide 3 to 7 parts cadmium chloride 0.0001 to 0.002 part copper as a copper salt,

and ring said coating yat a temperature in the range of about 51.0l to 550 C. in a restricted Volume of stagnant alr.

3. A process for producing a photoconducting layer comprising coating a substrate with an intimate mixture in parts by Weight of:

100 parts cadmium selenide 3 to 7 parts cadmium chloride 0.0001 to 0.002 part copper as` a copper salt,

heating said coating up to Ia temperature in the range of about 510 to 550 C. inabout 4to 8 minutes, and firing heating said coating up to a temperature in the range of about 510 to 550 C. in `about 4 to 8 minutes, firing said coating at a temperature in the range of about 510 to 550 C. `fofr `about 10 to 14 minutes` in a restricted volume of stagnant air and then cooling said tired layer in about 30 minutes. K

5. A process for producing a photoconducting layer comprising coating a dense, high-red, high-alumina ceramicsubstrate With an intimate mixture in parts by weight of:

parts cadmium selenide 5 parts cadmium chloride 0.0015 copper as copper chloride,

heating said coating up to a temperature in the range of about 510 to 550 C. in about 4 to 8 minutes, and firing `said coating at a temperature in the range of about 510 to 550 C. `for about 10 to 14 minutes in a restricted volume of stagnant 6. A process for producing a photoconducting layer comprising coating an alumina ceramic substrate with an intimate mixture in parts by weight of:

100 parts cadmium selenide 5 parts cadmium chloride 0.0015 copper las copper chloride,

heating said coating up to a temperature of about 530 in about 6 minutes, and tiring said coating at a temperature at 530 for about 12 minutes in a restricted volume of stagnant air.

7. A photoconductive layer made by the process of claim 3.

8. A photoconductive layer made by the process of claim 1.

References Cited in the le of this patient UNITED STATES PATENTS 2,765,385 Thomsen oct. 2, 1956 

1. A PROCESS FOR PRODUCING A PHOTOCONDUCTING LAYER COMPRISING COATING A SUBSTRATE WITH AN INTIMATE MIXTURE IN PARTS BY WEIGHT OF: 100 PARTS CADMIUM SELENIDE 3 TO 7 PARTS CADMIUM CHLORIDE 0.0001 TO 0.002 PART COPPER AS A COPPER SALT, AND FIRING SAID COATING IN A RESTRICTED VOLUME OF STAGNANT AIR UNTIL SAID MATERIAL IS SINTERED. 